A. Field of Invention
The present invention relates generally to diversity combining or voting to distinguish the most useable or preferred signal from a number of homogenous streams of transmitted audio content received at multiple receiving antennas from a subscriber radio or device of a wireless communication system, and, in particular, in conventional analog streams the ability to process signaling characteristics used for voting from a greater number of streams requiring digital signal processing than the number of available digital signal processors (“DSPs”), while maintaining audio quality of the voted stream.
B. Problems in the Art
Many wireless communication systems are comprised of a plurality of mobile (vehicle-mounted) or portable (hand-held) transceivers and a plurality of receivers positioned over an intended space of use. Due to power and size considerations inherent to them, these mobile or portable devices have a relatively small effective broadcast range when compared to stationary antennas of the receivers.
1. LMR and Space Diversity
One example is land-mobile radio (“LMR”). For LMR mobile or portable radios (subscriber units or subscribers) which communicate with a base station over a single frequency, area coverage (space diversity) in size and quality can be increased through the use of the multiple, geographically distributed receivers. These receivers are often located remote from the base station to extend quality coverage of the receiver end of a repeater system. Because the mobile or portable radio transmission can be received by multiple receivers simultaneously, diversity combining or voting is a well-known and widely used method to evaluate and select from these multiple signals to present a high quality, continuous version of the mobile radio transmission content for re-transmission to another mobile or portable subscriber radio or listening at, for example, a base station or dispatch console.
Even though the audio streams in the signals from the multiple receivers should have the same audio content (e.g. the same voice message), quality of the signal may vary significantly. Thus, signal quality is a conventional consideration when selecting or voting the “best” or preferred signal. For good reproduction of the audio content of the original call, the stream with the most desired characteristic(s) is selected from the set of homogenous streams to ultimately relay to the intended recipient(s) of the transmission.
2. Conventional Diversity Combining or Voting Diversity combining or stream selection uses the voting comparator or voter to analyze one or more signal characteristics of the streams and to choose a preferred or “voted” stream. In order to compare signal characteristics, the signaling information must be processed from the transmission. There are a variety of signals which are transmitted by mobile or portable radios, each of which has different processing requirements.
In the case of what is termed in the art as conventional analog (as opposed to purely digital), DSPs are used as an efficient way of processing the signaling information associated with each stream. One example is Continuous Tone-Coded Squelch System (CTCSS) selective calling. Therefore, many conventional analog systems rely on such digital signal processing in a DSP, even though the audio content is analog, e.g., analog frequency modulation (FM) or phase modulation (PM). Some conventional analog systems superimpose digital data on the analog content. DSP processing is also an efficient method of handling these signals. An example is Digital-Coded Squelch (DCS or sometimes called CDCSS). Others exist.
One function of the voting comparator is to discriminate between different received versions of the subscriber call. As described above, with space diversity multiple receiver/antennas, a high quality continuous version of the audio content of the call may not always be available through a single path from the subscriber.
Thus, a variety of selection methods for two or more signals by a voter or voting comparator have been developed and are known in the art. Just a few general examples can be found at U.S. Pat. Nos. 4,013,962 and 5,719,871, 6,321,086, which are incorporated by reference herein. The voting comparator performs an evaluation of all received signals related to a subscriber call and picks the most useable or preferred received signal. The voted audio content goes to a repeater and/or a console speaker at the base station and the audio from all other signals is either muted or ignored. A voting comparator can either pick the “best” of the signals and use that signal for the remainder of the call, or can pick the “best” signal and then switch between receivers in tenths or hundredths-of-a-second (faster than a single syllable) if the originally selected signal ceases to be “best”. Thus, voting is usually based on some characteristic of signal quality. In LMR, one commonly used criterion or voting metric is received signal strength indicator (“RSSI”). This can be sent with the audio stream from its receiver, sent separately but correlated to its audio stream, or measured and correlated with each stream at the receiver or voter.
Some communications systems, including some LMR systems, organize at least the signaling information related to each stream into digital data in what are referred to as containers, frames, or packets, each of which has control information (e.g. subscriber and receiver identities, priority, status so that each stream can be recognized and processed). Existing state of the art voters tend to rely on utilizing a single DSP for processing the data related to each of the streams the voter can receive. See simplified diagram of FIG. 1, showing one full decode DSP per audio stream to be voted. One advantage of this paradigm is that the full, decoded signaling content of each stream AS(1) to (N) is immediately available for selection at any time, and in essentially “real time”. This allows quick selection of a single high quality signal from the multiple streams, without loss of content or time lag, even if the voter switches between streams. However, using dedicated DSP modules for every possible analog stream a voter could receive represents a significant cost to a radio system as well as a severe limit on the amount of streams a voter can process concurrently.
Another problem related to radio transmission is that signal quality is often difficult to distinguish. There exist in the art a plurality of metrics used to characterize signal quality and distinguish between signals based on the same; however these various metrics have specific areas of strength and weakness that limit their usefulness in determining true signal quality. The state of the art tends to rely on just one metric. For example, RSSI tends to be a good indicator of signal strength, but signal strength may not equate with audio quality. RSSI also has variability dependent upon a number of factors. Out-of-band noise (OOBN) is a good metric for low signal quality, but can give false noise indications for signals above a certain dB SINAD (“signal to noise and distortion”). Signal-to-Noise Ratio (SNR) can take significant time to establish. Also, if established at the receiver, SNR ignores noise between the receiver and the voter. Therefore, improvement in signal quality metrics, including use of multiple or combined signal quality metrics, could give a more accurate measure of signal quality related to audio quality.
Furthermore, it is a well recognized problem that radio systems are often incompatible with each other. This can be problematic and dangerous, especially in the case of first responders and emergency personnel. A radio system with voting that is compatible with a number of communication protocols could represent an improvement in the art.
Therefore, there is a need for voting system that improves processing efficiencies and is economical by utilizing fewer digital signal processors than conventional. A need has been identified for a more economical, non-complex, practical way of using a limited number of DSP modules to process a larger number of audio streams without affecting the quality of the voted audio stream.